Optimal performance of digital radio receivers is achieved when the analog-to-digital (A/D) conversion of received analog signals is performed as close as possible to the receive antenna. While A/D conversion at the antenna is not commercially viable due to large bandwidth and high dynamic range requirements, conversion at the intermediate frequency (IF) of a superheterodyne receiver is possible. Conventional narrowband AID conversion techniques centered at 10.7 MHZ are known using complementary metal-oxide semiconductor (CMOS) and/or CMOS having on chip bipolar transistor (BiCMOS) technologies, however conventional signal conversion at much higher frequencies remain inadequate and at the very least require enhancement of existing CMOS-based A/D converter architectures.
IF frequencies for most wireless receivers typically range from 70 MHZ to 400 MHZ. If the received radio signal is converted to digital form early in the reception process, several benefits result. First, digital processing at an earlier stage permits a high degree of programmability in the filtering and demodulation process, thus easing compatibility with various wireless standards. Second, for in-phase/quadrature (I/Q) receivers, demodulation when performed digitally does not suffer degradation due to mismatches in the in-phase and quadrature channels as presently occurs with conventional analog I/Q demodulation techniques. Third, high frequency A/D conversion allows for improved manufacturability of a chip having both analog and digital circuitry because the system solution will have a higher percentage of digital circuitry. And finally, once the received signal is converted to a digital signal, logic circuits processing the data can operate at relatively low voltages to save power without suffering a loss in dynamic range as would be the case with conventional analog circuits.
Various digital receiver architectures have been devised of which a few are briefly described herein. A first conventional homodyne receiver translates the incoming radio frequency (RF) signal to baseband after limited filtering and amplification at RF. Channel selection and signal amplification are performed at baseband prior to the A/D conversion, and in-phase and quadrature paths are required to separate the image frequencies. A digital signal processor (DSP) is then required for signal demodulation, symbol timing recovery and maximum likelihood detection to yield the transmitted digital data. Four basic problems exist with this architecture: (1) mismatches between the analog I/Q paths limit image frequency separation, (2) the frequency of the local oscillator is identical to the desired input frequency and can inadvertently couple to the antenna and radiate thus causing interference, (3) large DC offset in the signal path can be generated thus reducing the available dynamic range of the receiver, and (4) the high dynamic range required of the baseband filters is extremely difficult to achieve with integrated circuit active filters.
To avoid these problems, the most common approach is to use a superheterodyne receiver. Instead of frequency translating the received analog signal to baseband after RF amplification, as with conventional homodyne receivers, a first local oscillator translates the spectrum to an IF frequency for purposes of channel selection and amplification. I/Q branches are then used in the demodulation process. Although performance is better as compared with homodyne receivers, I/Q mismatches in the analog signal path remain problematical. The difficulty with conventional superheterodyne receivers is obtaining an adequately high-quality factor (Q) bandpass filter and amplifier at IF frequencies. Such a high-Q, high dynamic range bandpass filter cannot presently be implemented with analog active filters in very large scale integration (VLSI) circuits, but is readily available using off-chip with surface acoustic wave filters.
Other conventional designs for performing A/D conversion at the IF frequency have shown to alleviate many of the aforementioned problems. For example, in a variation of the conventional superheterodyne receiver, mixers and I/Q paths are implemented digitally to eliminate frequency "leakage." Channel selection filters are also implemented as digital lowpass filters after A/D conversion of the received signal. Other designs further include an analog bandpass filter at IF frequencies to attenuate large out-of-band signals to limit the dynamic range requirements of the A/D converter. The bandpass filter serves the additional function of anti-aliasing the bandpass A/D converter, and as such the resulting lower filter Q eases the filter's frequency accuracy and noise requirements.
Conventional discrete-time bandpass sigma-delta (.SIGMA.-.DELTA.) modulators, for example, have been used for digitizing narrowband input signals centered at one fourth the sampling frequency, f.sub.s /4, of the A/D converter. See S. Jantzi, R. Schreier and M. Snelgrove, "The Design of Bandpass Delta-Sigma ADCs," Delta-Sigma Data Converters: Theory, Design, and Simulation, edited by S. Norsworthy, R. Schreier and G. C. Temes (IEEE Press 1997). Accordingly, analog filters resonating at f.sub.s /4 are used to suppress the quantization error in the desired frequency band. This however poses two primary problems that limit the resolution of discrete-time bandpass sigma-delta modulators when the desired center frequencies are in the order of 10's of MHZ's.
A first problem is due to resonant frequency errors caused by capacitor mismatches in switched-capacitor implementations of conventional discrete-time bandpass .SIGMA.-.DELTA. modulators. These mismatches cause significant quantization noise to appear in the signal band, and as such degrade the performance of the .SIGMA.-.DELTA. modulator. N-Path filtering techniques have been shown to eliminate resonant frequency errors in switched capacitor and other sampled data implementations. See R. Schreier and G. C. Temes, "Multibit Bandpass Delta-Sigma Modulators Using N-Path Structures," IEEE International Symposium on Circuits and Systems, pp. 593-596 (1992). However, resonant frequency errors remain problematical for continuous-time resonators. Integrated active continuous resonators would require sophisticated self-tuning of the resonant frequency.
Second, in sampled-data resonators, the front-end circuitry must sample and hold the input waveform to an accuracy exceeding the A/D requirements. For example, converting narrowband signals centered at 70 MHZ with the passband of the A/D converter centered at f.sub.s /4 having an accuracy of 12 bits would require the sample-and-hold (S/H) circuitry to sample at 280 MHZ with an accuracy exceeding 12 bits, e.g., 13 bits. These are extremely difficult requirements for any conventional integrated circuit technology, especially CMOS.
Although conventional continuous-time bandpass .SIGMA.-.DELTA. converters can operate at very high sample rates without the front-end S/H circuitry, accurately controlling the center frequency and Q of conventional continuous-time bandpass .SIGMA.-.DELTA. converters can be very difficult. These converters require high-Q resonators in order to obtain adequate quantization error rejection and avoidance of dead zones.
Also, in continuous time .SIGMA.-.DELTA. modulators, the shape of the digital-to- analog pulse feedback to the input of the converter can limit resolution if excessive sampling jitter or high frequency noise is present. As such, careful control of the feedback signal characteristics is required to control the modulator's stability and dynamic range.
Therefore, it is a principle object of the present invention to provide a .SIGMA.-.DELTA. modulator having minimal increase of passband quantization noise due to resonant center frequency errors.
It is another object of the present invention to provide a .SIGMA.-.DELTA. modulator having a continuous-time resonator with moderate Q and center frequency accuracy requirements.
It is yet another object of the present invention to provide a .SIGMA.-.DELTA. modulator for the direct conversion of radio or intermediate frequencies to baseband or other low or intermediate frequencies for use in wireless communication systems and other digital receiver systems.
Further objects, features and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures showing illustrative embodiments of the invention.